In the Laboratory
Inserting an Investigative Dimension into Introductory Laboratory Courses Carolyn Herman Department of Chemistry, Southwestern College, Winfield, KS 67156 There is broad consensus that science education needs to engage students as active learners. This appraisal is essentially Piagetian and assumes that knowledge is constructed in the mind of the learner, based on the learner’s experience (1–3). Many strategies can be synergistically employed to attain this goal, including computer simulations, discussions of lecture demonstrations, cooperativelearning assignments, case studies, marathon problems, undergraduate research, and laboratory exercises that provide students opportunities to investigate phenomena. Including investigative laboratories in the introductory curriculum is especially rewarding: in addition to enhancing student learning, they more accurately represent the true nature of the scientific enterprise. There are at least two strategies for enhancing the investigative nature of laboratory: (i) providing instructions for performing the experiment, but leaving the analysis of data or hypothesis formation open-ended; and (ii) requiring students to design their own experiments. The Abraham and Pavelich laboratory manual is an excellent example of the first strategy (4). Clear instructions for how to collect data are included in each laboratory exercise. The data analysis is quite open-ended, however. Students are asked to look for patterns in data and to form hypotheses based on patterns; they are then sometimes directed to collect additional data in order to test the hypotheses developed. The second strategy in also used to a limited extent in the Abraham and Pavelich text. At some institutions, however, adopting a new text may not be desirable or practical. This article will discuss how to convert traditional laboratories into investigative laboratories by asking students to design their own procedures. The following considerations are useful in identifying what labs can be easily reconfigured to this format and in determining exactly how to accomplish that restructuring. 1. Which concepts that the laboratory exercise teaches are most important? Can these concepts be rephrased as questions that can be answered experimentally, without turning the lab into a mere verification of known information? 2. Can freshmen be expected to understand the experimental goal well enough to design a reasonable experiment? 3. What knowledge is essential before students can collect meaningful data? Can this information be provided as stand-alone background material?
Verifying vs. Investigating the Ideal Gas Constant As an example, measuring the ideal gas constant frequently is assigned in cookbook-style lab manuals. First, what are the important concepts? The central idea is the ideal gas constant and the other variables in the ideal gas equation. The question traditionally pursued is, “What is the value of the ideal gas constant?” This is a verification exercise and it generally does not work well for freshmen. This leads to two negative outcomes: students think that the goal of an experiment is to get a predetermined right
70
answer, and they are frustrated because they cannot get this right answer. We can rephrase the question as, “How could we measure the relationship between pressure, temperature, volume, and number of moles?” This transforms the experiment into a genuine search for knowledge that cannot easily be found in the text: what works experimentally and what does not? Second, can freshmen be expected to understand the experimental goal well enough to design a reasonable experiment? The key in this example is the four variables: temperature, pressure, volume, and number of moles. All of these variables are concrete, and students already have practice measuring several of them. Third, what background knowledge is essential for students to be able to design a reasonable experiment? Obviously, they need some introduction to the ideal gas equation. Students can reason from their own experience about relationships between temperature and pressure in a fixed volume (an aerosol can explodes when heated), temperature and volume at a fixed pressure (a balloon that is heated expands), and moles of gas and volume at fixed temperature and pressure (the volume of a balloon expands when more gas is added). These relationships can be combined and expressed as a proportion before the laboratory. Students also need to know what unfamiliar equipment and materials are available. Can they measure pressure directly, or does the system need to be open to the atmosphere so that the barometric pressure can be used? Gas behavior is usually covered after moles and volume are relatively familiar concepts, but absolute temperature may be a new idea, that must be introduced as part of the background information for the laboratory. Students will also need to know what gases are available for use in the experiment. Will canisters of fairly dense gases like carbon dioxide be available? Are students allowed to perform and then trap the products from a reaction that will generate a gas? If so, have they already worked with a reaction that produces one or more gases in lab, or will they need some example reactions demonstrated? Will low-boiling-point liquids like ethanol or cyclohexane be available? If so, will students need to use a CRC handbook to look up boiling points, or will selected chemical data be more readily available to them? Some students may not know that temperature at the boiling point is constant until all liquid is vaporized. In addition to appropriate background information, they also need to have their proposed procedures reviewed by someone experienced who can help them correct major errors in reasoning or dangerous procedures. In first-semester laboratory courses where I have used this approach, students have designed a variety of experiments. Some set up a system for collecting gas over water that is similar to a diagram in their lecture text, using a gas-producing reaction with which they are already familiar from previous experiments or lecture demonstrations. Others boil away a small quantity of a volatile liquid in a container open to the atmosphere until the temperature first begins to rise above the boiling point, then recondense the substance and weigh how much remains in the con-
Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
In the Laboratory tainer. Some students vaporize a substance in a closed container and use a manometer to measure the pressure. The pedagogical strategy described here necessarily builds on students’ previous experience. For example, one group knew from high school that iodine vaporizes at low temperatures and therefore used iodine in their experiment. It worked well, since a relatively small number of moles corresponds to a mass that can be weighed with reasonable precision on a standard freshman chemistry balance. Another group contained a member who knew that metal carbonates release carbon dioxide upon heating. They filled a balloon with the reaction products, measured the mass difference of the reactants and determined the volume of the gas by submerging the balloon in water. Their first mass difference was very small, forcing them to reevaluate their original experimental design. They eventually chose to use an analytical balance for mass determinations, along with a larger balloon for gas collection. Since some students in each lab collect a gas over water, correcting their data for the partial pressure of water vapor as a class exercise is a concrete segue to a discussion of Dalton’s law. Determination of a True Unknown by Investigating Physical Properties Students with very little laboratory experience are capable of designing adequate experiments that they understand thoroughly. Let us consider an exercise that is often performed in the first week or two of lab, determination of an unknown by examination of physical properties. What is the central concept? Different elements and compounds have different physical and chemical properties. The question usually addressed is, “The unknown is one of the compounds listed in this chart of melting points, boiling points, densities, and solubilities. Which is it?” A nonverification version of this question might involve a genuine unknown and be phrased as, “This is some inorganic substance that has been improperly labeled. What is it, so we can use it or dispose of it properly?” Can students grasp the experimental goal? It is not news to students that different substances have different characteristic colors, odors, etc. This real-world observation can easily be extended to chemical and physical properties that are tabulated in reference texts. What background knowledge is essential for students to be able to design a reasonable experiment? If a real unknown is used, the format of the CRC handbook and the meaning of tabulated properties need to be explained. The instructor should determine whether the unknown is organic or inorganic. Students then need to know what capabilities are available. What is the melting/boiling point range that can be investigated? What solvents are allowed for solubility tests? Is an osmometer available to determine the molecular mass? Once students know what their resources are, they can choose which properties they wish to investigate and in what order. By comparing their data to the listings in the CRC handbook, a process of elimination may enable them to make an unequivocal determination of the identity of the unknown, or they may only be able to submit a list of possible unknowns. In the latter case, the sample can be used later as a qualitative analysis unknown. In our freshman chemistry laboratory, the source of genuine unknowns has been a supply of bottles and jars left by my predecessor with labels such as “unknown hydrate 4” and “boiling point unknown A.” If no one in your department has similar bottles and jars, a local high school may
be a source of true unknowns that can be presumed safe for student handling. Art departments are also a possible source of unknowns, although the instructor should determine whether or not they are mixtures before assigning them for class analysis. Experiments That Are Difficult To Restructure In what sorts of experiments is it difficult for students to design their own procedures? With some creativity, the underlying question in most experiments can be rephrased as a genuine inquiry rather than a verification exercise. When experiments do not lend themselves well to this framework, generally the experimental goal is obscure to freshmen or the required background information is too complex. An example is iodometric titration. The reactions upon which the results depend are complicated. Many freshmen get lost in the details of the specific reactions and are unable to think clearly about the “big picture”. This makes design of a good experiment nearly impossible. Although qualitative experiments in kinetics are quite accessible (e.g., what factors influence reaction rate?), quantitative kinetics experiments are another category of standard labs that are difficult for first-year students to perform independently. Without some idea of what sort of mathematical relationship they are looking for, most students will flounder and are unlikely to design good strategies for collecting useful data. Even if the experimental goal is more concrete, sometimes the experimental structure is just too daunting. One example is spectrophotometric determination of the formation constant for Fe(III) ions and thiocyanate ions. If the multiple equilibria between Fe3+ , SCN {, [Fe(SCN)]2+, and [Fe(SCN)2 ]+ are simplified to a single equilibrium expression, the experimental goal, measuring the concentration of the colored complex in comparison to known initial concentrations of iron and thiocyanate, is reasonably clear. However, the necessary background information requires a fairly firm grasp of the central concept in the experiment. Most cookbook lab manuals direct students to establish a standard curve by reacting a huge excess of iron with varying concentrations of thiocyanate. The excess of iron drives complex formation, and the concentration of the iron– thiocyanate complex at equilibrium can be assumed to be stoichiometrically equal to the initial concentration of thiocyanate. To design a procedure for obtaining a standard curve, however, requires a fairly sophisticated comprehension of equilibria. If students are learning about equilibria, they are unlikely to correctly measure the relationship between absorbance and complex concentration. Summary Despite widespread support for the concept of investigative laboratories at the introductory level (5–9), only limited change is observed in college and university chemistry programs (10). This is partly because restructuring laboratories presents difficult administrative issues. Teaching assistants for investigative laboratories require more sophisticated training than graduate students have traditionally received. Introducing a project-oriented format increases the preparation time for reagents and materials and requires vigilant safety rule enforcement. These major practical issues suggest that most colleges and universities will be more successful at changing the laboratory in increments rather than introducing sweeping changes in the lab curriculum.
JChemEd.chem.wisc.edu • Vol. 75 No. 1 January 1998 • Journal of Chemical Education
71
In the Laboratory One approach to achieving incremental change is to rework a laboratory exercise that is already a part of the curriculum. Not every traditional introductory lab exercise can be easily converted to a genuine experiment that students plan and then conduct, nor is student design of their own experiments the only strategy for enhancing the investigative component of teaching laboratories. Some experiments that cannot be restructured may still have immense pedagogical value and should not be discarded. Nevertheless, some laboratory activities in a standard freshman curriculum can be transformed into genuine inquiries. By slowly introducing appropriate investigative components into a structure that is already comfortable and working smoothly, advocates of change reduce anxiety in the department. Transforming one lab at a time also reduces new training for graduate assistants to a manageable level. If one lab per semester or year is adapted to an investigative format, a balance eventually can be reached between open-ended laboratory experiences and more traditional opportunities to practice complex experimental techniques.
72
Literature Cited 1. 2. 3. 4. 5. 6.
7. 8. 9.
10.
Herron, J. J. Chem. Educ. 1975, 52, 146–149. Herron, J. J. Chem. Educ. 1978, 55, 165–170. Bodner, G. J. Chem. Educ. 1986, 63, 873–878. Abraham, M. R.; Pavelich, M. J. Inquiries into Chemistry, 2nd ed.; Waveland: Prospect Heights, IL, 1991. Prendergast, A. The Liberal Art of Science; American Association for the Advancement of Science: Washington, DC, 1990. Yankwich, P. Tomorrow. The Report of the Task Force for the Study of Chemistry Education in the United States; American Chemical Society: Washington, DC, 1984. National Science Foundation. J. Coll. Sci. Teach. 1990, 19, 134– 139, 146–147. Criteria For Excellence. An NSTA Science Compact; National Science Teachers Association: Washington, DC, 1987. Harrison, A. An Exploration of the Nature and Quality of Undergraduate Education in Science, Mathematics and Engineering, A Report of the National Advisory Group of Sigma Xi; Sigma Xi, The Scientific Research Society: Research Triangle Park, NC, 1989. Abraham, M. R.; Cracolice, M. S.; Graves, A. P.; Aldahmash, A. H.; Kihega, J. G.; Palma Gil, J. G.; Varghese, V. J. Chem. Educ. 1997, 74, 591–594.
Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
